Saturation spectroscopy is a well-known technique with a variety of uses. In many applications it is used to compare the frequency of a laser to the frequency of an atomic transition in a dilute gas. For instance, in general the frequency of a laser is not fixed, but will drift due to thermal effects, aging, and other time dependent processes. Some applications (such as cold atom devices, including atomic clocks) require that the laser frequency be controlled by a feedback system to prevent this drift.